The present disclosure relates to a method for operating an engine in an intermittent combustion mode and an engine control device.
U.S. Pat. No. 7,577,511 discloses a method for executing an intermittent combustion mode in which combustion in cylinders is intermittently skipped. The publication discloses a method for adjusting the engine output by changing the ratio of fired cylinders γ [γ=the number of fired cylinders/(the number of fired cylinders+the number of skipped cylinders)] in the intermittent combustion mode.
In the above publication, the fired cylinder ratio is set to 6/8 (=75%) by executing the intermittent combustion mode with a pattern in which five cylinders are successively fired, one cylinder is skipped, one cylinder is fired, and then one cylinder is skipped. In this intermittent firing pattern, a period corresponding to five cylinders and a period corresponding to one cylinder exist as a skipped-cylinder interval. The skipped-cylinder interval is represented by the number of cylinders that are fired from when the combustion is skipped until another combustion is skipped.
In a period in which the skipped-cylinder interval is long, the generation amount of torque per unit time is increased. In a period in which the skipped-cylinder interval is short, the generation amount of torque per unit time is decreased. For this reason, if there are periods in which the skipped-cylinder intervals differ greatly in the intermittent firing pattern, the rotational fluctuation of the engine is increased.
In contrast, if the skipped-cylinder intervals are constant, the torque fluctuation caused by skipping the cylinders occurs in a certain cycle, which generates vibration and noise. Thus, vibration and noise having low frequencies that are likely to disturb the occupant may possibly occur.